I too have had this fault hundreds of times!!!
I was told by ford mechanic that it is a design fault.
The terrain management system is dependent on sensors in the wheels that tell the system which are sticking and slipping. When anything (rain,dust,mud,lint,snow,grass,etc...) gets into these sensors it causes the fault, rendering your 4WD a 2WD. Also causes several other things to not work including: adaptive cruise control, collision avoidance, hill decent, traction control, and a few more.
When the fault happens it beeps at you several times and continues to do this even after you acknowledge the faults, really annoying!!!
Sometimes the fault goes away by turning off and on, but if you have mud or something more permanent in the sensor it will no go away until you take to a hoist and lift up vehicle so that you can pressure wash the back of the tires which is the only way to get at these sensors. To do this ford charges me $120! Each time, I can't afford this twice a week so now just put up with fault msgs
I am glad that others are seeing this issue, ford tells me that I am the only one, an that it is because I live in the country with gravel roads! WTF I bought this vehicle so my wife would have a 4WD BECAUSE we live in the country.
Ford has made a 4WD that does not work in snow, mud, or any other substance where you need 4WD
I was told by head mechanic for ford that this is a design flaw. Ford will not fix as it is not happening to enough people.